1. Field of the Invention
This invention relates to new photoresist compositions especially for deep U.V. (particularly 248 nm) and I-line (365 nm) exposures and having the capability of forming highly resolved features.
2. Background Art
Photoresists are photosensitive films for transfer of images to a substrate. They form negative or positive images. After coating a photoresist on a substrate, the coating is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the photoresist coating. The photomask has areas opaque and transparent to activating radiation that define a desired image to be transferred to the underlying substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresists is generally described, for example, by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principals, Practices and Materials, Plenum Press, New York (1988).
An important property of a photoresist is image resolution. A developed photoresist image of fine line definition, including lines of sub-micron and sub-half micron dimensions and having vertical or essentially vertical sidewalls is highly desirable to permit accurate transfer of circuit patterns to an underlying substrate. However, many current photoresists are not capable of providing such highly resolved fine line images.
For example, reflection of activating radiation used to expose a photoresist often poses limits on resolution of the image patterned in the photoresist layer. Reflection of radiation from the substrate/photoresist interface can produce variations in the radiation intensity in the photoresist during exposure, resulting in non-uniform photoresist linewidth upon development. Radiation also can scatter from the substrate/photoresist interface into regions of the photoresist where exposure is not intended, again resulting in linewidth variations. The amount of scattering and reflection will typically vary from region to region, resulting in further linewidth non-uniformity.
Variations in substrate topography also give rise to resolution-limiting reflection problems. Any image on a substrate can cause impinging radiation to scatter or reflect in various uncontrolled directions, affecting the uniformity of photoresist development. As substrate topography becomes more complex with efforts to design more complex circuits, the effects of reflected radiation become more critical. For example, metal interconnects used on many microelectronic substrates are particularly problematic due to their topography and regions of high reflectivity.
With recent trends towards high-density semiconductor devices, there is a movement in the industry to shorten the wavelength of exposure sources to deep ultraviolet (DUV) light (300 nm or less in wavelength) including excimer laser light (ca. 248 nm) and ArF excimer laser light (193 nm). The use of shortened wavelengths of light for imaging a photoresist coating has resulted in greater penetration of the photoresist layer and increased reflection of the exposing energy back into the photoresist layer. Thus, the use of the shorter wavelengths has exacerbated the problems of reflection from a substrate surface.
Many existing exposure tools are designed for imaging with relatively longer wavelengths, e.g. I-line (ca. 365 nm) wavelengths. However, advanced resists designed for imaging at shorter wavelengths such as 248 nm often can not be effectively exposed with an I-line exposure because the resist's photoactive component is not activated at 365 nm or other higher wavelengths. This requires design and supply of additional resist compositions that can be effectively imaged at selected wavelengths.
It thus would be desirable to have new photoresist compositions that could provide highly resolved fine line images, including images of sub-micron and sub-half micron dimensions. It would be further desirable to have such new photoresist compositions that could be imaged with deep U.V. radiation as well as other wavelengths, particularly 365 nm. It would be particularly desirable to have such photoresists that reduce undesired reflections of exposure radiation.